With advancing technology comes the increased use of non-linear loads on electrical power distribution systems. This, in turn, results in unwanted distortion of the system voltage, i.e. poor power quality. Both airborne and ground based power systems are victims of this trend. The task of specifying input current characteristics for these loads to eliminate the problem becomes a formidable, if not impossible, task.
An alternate solution is to connect a separate electronic box(es) to locally correct the voltage distortion on the system regardless of the source of the problem. In this manner a power system designer can optimize the system in terms of cost, weight, performance and schedule without imposing on the designer of every piece of user equipment connected to the power distribution grid. Historically, this would be accomplished by adding a capacitor to the power grid to act as a local filter. This approach carries with it a reactive (leading power factor) current which can create other problems such as generator self-excitation and subsequent loss of control if the system power factor becomes sufficiently leading.
To reduce the risks associated with the use of local capacitive filtering, systems employing active filters, such as that disclosed by U.S. Pat. No. 5,397,927 awarded to Suelzle et al. for an "Active Filter for Reducing Non-fundamental Currents and Voltages", may be used. This system requires that each of its' control signals, both voltage and current, be filtered by a notch filter to remove the fundamental frequency component of the bus waveform and leave only the harmonic and other non-fundamental frequency currents. These control signals control the generation of an injected current component proportional to these harmonic and other non-fundamental frequency currents and an injected current component proportional to the non-fundamental frequency line voltages but opposite in phase with them into the system.
The requirement of notch filtering to remove the fundamental frequency component, however, adds cost and complexity and the reduces reliability of the system. In addition, this requirement reduces the performance of the active filter itself if the frequency is allowed or is required to deviate from the tuned notch frequency of the control signal filter. A typical fixed frequency power distribution system for use on aircraft, for example, allows a 40 Hz deviation in bus voltage. Additionally, a system such as disclosed by U.S. Pat. No. 5,397,927 would have particular performance problems in variable frequency systems where the fundamental frequency of the system is allowed to vary over a range of approximately 2:1.
It is an object of the instant invention, therefore, to provide a harmonic distortion compensator for application as a local area compensation on a power generation and distribution system or as a dedicated load filter for a non-linear ac load. Further, it is an object of the instant invention to provide a harmonic distortion compensator which will operate over a wide fundamental frequency range and which will appear as a virtual no-load to the system. Additionally, it is an object of the instant invention to provide a distortion compensator or `scrubber` which may also be used in conjunction with a low quality source to provide good power quality to the distribution loads.